This application relates to a system and method for regulating the temperature of a self-contained fuel cell apparatus preferably comprising a fuel reformer. The invention maintains the various components of the fuel cell apparatus within preferred operating temperature ranges while ensuring that exhaust gases and external surfaces of the apparatus do not exceed safe temperature levels. The invention is particularly suited for self-contained hybrid power applications.
Fuel cell systems, particularly those comprising fuel processors, generate significant heat at elevated temperatures. For example, conventional methanol reformers operate at temperatures on the order of 400xc2x0 C. In order maximize the efficiency of the reforming and electrochemical processes, and the useful life of system components, the excess heat must be effectively extracted and dissipated. Operating fuel cell systems efficiently in very cold ambient temperature environments poses other thermal management challenges, particularly in the case of hybrid systems comprising temperature sensitive storage batteries.
In most fuel cell systems the component parts are liquid-cooled. Such conventional systems require the use of conduits to direct coolant into thermal contact with the fuel cells. The fuel cell stack usually includes a manifold and inlet for directing coolant fluid, typically water, to the interior of the stack to absorb heat generated by the exothermic reaction of hydrogen and oxygen within the fuel cells. In many cases heat extracted from the system is transferred to a thermal load for co-generation purposes.
U.S. Pat. No. 4,578,324, Koehler et al., issued Mar. 25, 1986 typifies prior art fluid cooled systems. The cooling system comprises cooling panels arranged adjacent to the electrochemical cells of a fuel cell stack. The cooling fluid may be circulated through a heat exchanger for disposing of excess heat before returning the fluid to a pump. The pump and heat exchanger are located external to the fuel cell.
U.S. Pat. No. 4,706,737, Taylor et al., issued Nov. 17, 1987, similarly discloses a fuel cell coolant inlet manifold and system including cooling plates disposed in the fuel cell stack. Cooling water is delivered to the stack through a manifold which communicates with the cooling plates. An outlet manifold and means for circulating the water through a coolant loop are also described. In U.S. Pat. No. 6,080,502, Nolscher et al., issued Jun. 27, 2000, a fluid-cooled fuel cell system is described comprising strategically positioned coolant distribution ducts to achieve uniform cooling of the fuel cell stack.
Some air-cooled fuel cell systems are known in the prior art, particularly in the case of low power applications. U.S. Pat. No. 5,470,671, Fletcher et al., issued Nov. 28, 1995, describes an electrochemical fuel cell employing ambient air as the oxidant and coolant. The fuel cell assembly may include a fan for directing ambient air onto the exposed surface of the cathode. The heat generated in the assembly is dissipated to the atmosphere through a thermally conductive plate. U.S. Pat. No. 5,645,952, Lampinen et al., issued Jul. 8, 1997, similarly discloses means for cooling a fuel cell assembly by circulating air between the electrochemical cells.
Prior art air-cooled fuel cell systems typically comprise air conducting channels for dissipating waste heat from the fuel cell to the environment in a direct fashion. The prior art does not address the problem of circulating air streams relative to other system components, such as the components of a hybrid power supply arrangement, in the most efficient manner to best utilize the cooling capacity of the air and minimize parasitic loads.
Functionally self-contained fuel cell systems pose particular design challenges. By way of example, the applicant has developed a hybrid power supply apparatus particularly adapted for battery replacement applications which may be used to power non-road electric vehicles, such as lift trucks, sweepers and scrubbers and ground support equipment. The hybrid apparatus may be substituted for conventional traction batteries in a xe2x80x9cplug and playxe2x80x9d manner without requiring any modification to the electric vehicle, or other load having relatively low power requirements. The apparatus is effectively self-contained since the only interface with the electrical vehicle is by way of a standard electrical DC connection (e.g. no inlets or outlets for circulating liquid coolant derived from the vehicle or some other external source are provided). Moreover, in order to ensure plug and play functionality and avoid the need for vehicle modification, no thermal load is available for transfer to the vehicle. Consequently all heat transfer must be with the surrounding environment only. Further, due to the proximity of the vehicle operator to the hybrid power supply apparatus during normal operation of the vehicle, ergonomic considerations require that the temperature of the exhaust gas stream and the external surfaces of the apparatus remain below safe thresholds.
The need has accordingly arisen for a system and method for regulating the temperature of a self-contained fuel cell apparatus, such as a hybrid power supply for battery replacement applications, which is constrained to operate within a small physical space. The invention maintains the various heat-generating components of the fuel cell apparatus within preferred operating temperature ranges while ensuring that exhaust gases and external surfaces of the apparatus do not exceed safe temperature levels.
In accordance with the invention, a method of regulating the thermal characteristics of a self-contained fuel cell apparatus is described. The apparatus may comprise, for example, a hybrid power supply apparatus having external surfaces and a plurality of heat-generating components housed within the apparatus, each of the components having different preferred operating temperature ranges. The method comprises the steps of:
(a) introducing a heat transfer gas into the apparatus;
(b) moving the heat transfer gas within the apparatus in one or more flow paths between the components to maintain the components within the preferred operating temperature ranges, whereby the flow paths are configured such that the heat transfer gas has sufficient cooling capacity to accept waste heat from each of the components located downstream therefrom; and
(c) exhausting the gas from the apparatus to the environment surrounding the apparatus,
wherein the temperature of the external surfaces and the gas exhausted from the apparatus is maintained below 70xc2x0 C.
Preferably the method involves the use of multiple flow paths and the step of transferring heat from heat transfer gas moving through a first one of the flow paths to heat transfer gas moving through a second one of the flow paths. At least some of the flow paths are preferably merged prior to exhausting the heat transfer gas from the apparatus. In one embodiment at least some of the heat transfer gas is recirculated within the apparatus to pre-heat the intake air.
In a preferred embodiment the heat transfer gas is air introduced into the apparatus through an inlet in communication with the environment. In one embodiment the air is introduced into the apparatus through a single inlet and exhausted from the apparatus through a single outlet. Preferably the air is exhausted at a temperature below 50xc2x0 C.
The various components of the apparatus may comprise a fuel cell, a fuel processor, such as a reformer, a DC/DC converter and an energy storage device, such as a battery. In this embodiment the method may comprise the steps of:
(a) moving the heat transfer gas in a first one of the flow paths from the inlet past the energy storage device and the DC/DC converter;
(b) moving the heat transfer gas in a second one of the flow paths from the inlet through the reformer; and
(c) moving the heat transfer gas in a third one of the flow paths from the inlet through the fuel cell.
The invention may further comprise the step of mixing the heat transfer gas from the second and third flow paths downstream from the reformer to dilute exhaust expelled from the reformer. Preferably the recirculated air does not contain any reformer exhaust. Advantageously, some heat from the heat transfer gas may be transferred to a fuel storage chamber of the apparatus.
In one arrangement, the heat transfer gas is moved through the first flow path downstream from the DC/DC converter to accept radiant heat from the reformer. The method includes the step of transferring heat from the heat transfer gas to a source of fuel for the apparatus prior to introduction of the fuel into the reformer. The heat transfer gas moving in the third flow path may comprise oxidant gas reacted in the fuel cell.